Field of the Invention
This invention relates to laser machining tubing to form stents.
Description of the State of the Art
This invention relates to laser machining of devices such as stents. Laser machining refers to removal of material accomplished through laser and target material interactions. Generally speaking, these processes include laser drilling, laser cutting, and laser grooving, marking or scribing. Laser machining processes transport photon energy into a target material in the form of thermal energy, photochemical energy, or both. Material is removed by melting and blow away, direct vaporization/ablation, or by formation of a plasma with ablation.
When a substrate is laser machined energy is transferred into the substrate. As a result, a region beyond the cutting edge is modified by the energy, which affects the properties in this region. This region may be referred to as the “laser affected zone” (LAZ). In general, the changes in properties in this region are adverse to the proper functioning of a device that is being manufactured. Therefore, it is generally desirable to reduce or eliminate energy transfer beyond removed material, thus reducing or eliminating the extent of modification and size of the region affected.
One of the many medical applications for laser machining includes fabrication of radially expandable endoprostheses, which are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel.
A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices, which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success.
The treatment of a diseased site or lesion with a stent involves both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent through a bodily lumen to a region, such as a lesion, in a vessel that requires treatment. “Deployment” corresponds to the expanding of the stent within the lumen at the treatment region. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into a bodily lumen, advancing the catheter in the bodily lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen.
In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon. The stent is then expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn. In the case of a self-expanding stent, the stent may be secured to the catheter via a retractable sheath or a sock. When the stent is in a desired bodily location, the sheath may be withdrawn which allows the stent to self-expand.
The structure of a stent is typically composed of scaffolding that includes a pattern or network of interconnecting structural elements often referred to in the art as struts or bar arms. The scaffolding can be formed from wires, tubes, or sheets of material rolled into a cylindrical shape. The scaffolding is designed so that the stent can be radially compressed (to allow crimping) and radially expanded (to allow deployment).
Stents have been made of many materials such as metals and polymers, including biodegradable polymeric materials. Biodegradable stents are desirable in many treatment applications in which the presence of a stent in a body may be necessary for a limited period of time until its intended function of, for example, achieving and maintaining vascular patency and/or drug delivery is accomplished. Due to their temporary nature, biodegradable stents are often referred to as scaffolds.
Stents can be fabricated by forming patterns on tubes or sheets using laser machining. Even though the basic laser-material interaction is similar, there are certain aspects that differ among types of materials (such as metals, plastics, glasses, and ceramics), i.e. different absorption characteristics. The properties of biodegradable polymers tend to be very sensitive to energy transfer from laser machining, depending on the laser wavelength and power. It is critical when forming a biodegradable polymer stent from a biodegradable construct using laser machining that damage to the polymer material of the stent pattern from the laser energy is minimized. This requires judicious selection of laser parameters that allow cutting of the polymer in a fast efficient manner while minimizing damage to the polymer.
It has been found that selecting a combination of parameters such as pulse energy, wavelength, and laser pulse duration is critical in defined the optimal process conditions for the type of material. Short pulse lasers can operate in a regime where absorption of energy by the polymer is via a multi-photon process which generates a plasma plume. An advantage of this process is formation of a small or thin LAZ. However, even with the optimal parameters the cutting process can be slower than desired with certain constructs, for example, constructs with thick walls that are used for making peripheral scaffolds. In such situations, modifying the laser parameters to increase cutting speed can result in undesirable damage to the construct material which can adversely affect scaffold performance. In some situations, multiple passes of the laser over the polymer are required to cut completely through leading to long processing times. Therefore, methods are needed for increasing laser machining speed without adversely affecting the material of as fabricated scaffolds.